Who do you think is the thief?

From the text, we can infer the doctor is the thief (or his gatekeeper).

The doctor shrugged, and his wet eyes never left Kino's eyes. He knew the pearl would be buried in the house, and he thought Kino might look toward the place where it was buried. "It would be a shame to have it stolen before you could sell it," the doctor said, and he saw Kino's eyes flick involuntarily to the floor near the side post of the brush house.